Grant Sanderson savors the surprisingly difficult problem 2 from the 2011 International Mathematical Olympiad and its surprisingly easy to understand solution. pdf 
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YOUTUBE M64HUIJFTZM Published on Aug 4, 2019.
The problem is interesting. The solution and solution process compelling. The distinction between "social" and "mathematical" aspects truly insightful. Appreciative viewers quote Grant's better lines.
“I know what you’re thinking --those do happen to be all prime numbers”
"Always start simple!"
“You want to believe a result before you try too hard to prove it”
"look for an invariant"
"If a made up windmill helps you prepare for a real problem, who cares if it's a fiction"
"Knowing when the math is hard is way harder than the math itself..."
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⇒ Lisa Sauermann: 2011 IMO page ⇒ Science Olympiad ⇒ Code Fragment